The present invention relates to a combustion method for NOx reduction, as well as an apparatus therefor, to be applied to water-tube boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NOx generation, there have been known (1) suppressing the temperature of flame (combustion gas), (2) reduction of residence time of high-temperature combustion gas, and (3) lowering the oxygen partial pressure. Then, various NOx reduction techniques to which these principles are applied are available. Examples that have been proposed and developed into practical use include the two-stage combustion method, the thick and thin fuel combustion method, the exhaust gas recirculate combustion method, the water addition combustion method, the steam jet combustion method, the flame cooling combustion method with water-tube groups, and the like.
With the progress of times, NOx generation sources even of relatively small capacity such as water-tube boilers have been coming under increasingly stricter regulation of exhaust gas, and so further reduction of NOx are demanded therefor. The present applicant proposed a NOx reduction technique for these demands by Japanese Patent Laid-Open Publication HEI 11-132404 (Specification of U.S. Pat. No. 6,029,614).
This prior art technique is intended to achieve NOx reduction by a combination of suppression of combustion gas temperature with water tubes and suppression of combustion gas temperature with exhaust gas recirculation. However, the technique was capable of NOx reduction up to only about 25 ppm, other than one that allows NOx reduction to below 10 ppm to be achieved. It is noted that NOx reduction with the value of NOx generation being not more than 10 ppm will hereinafter be referred to as super NOx reduction.
In this prior art technique, it is conceivable to enhance the function of combustion-gas-temperature suppression with water tubes with the aim of achieving the super NOx reduction. This functional enhancement is to provide water tubes in contact with a burner or to increase the heat transfer surface of water tubes. However, excessive fulfilment of this functional enhancement would cause an increase in pressure loss or an unstable combustion such as oscillating combustion.
Further, it is also conceivable to enhance the function of combustion-gas-temperature suppression with exhaust gas recirculation to achieve the super NOx reduction. This functional enhancement is to increase the exhaust-gas recirculation quantity. However, this functional enhancement would cause an amplification of unstable characteristics of exhaust gas recirculation. That is, the exhaust gas recirculation has a characteristic that exhaust-gas flow rate or temperature changes due to changes in combustion quantity or changes in load. Increasing the exhaust-gas recirculation rate would cause these unstable characteristics to be amplified, so that stable NOx reduction could not be achieved.
Furthermore, the functional enhancement for exhaust gas recirculation would cause the combustion reaction to be suppressed, which would lead to an increase in emission of CO and unburnt components as well as to an increase in thermal loss. Also, increasing the exhaust gas recirculation rate would cause the blower load to increase. Excessive suppression of burning reaction would lead to an increase in emission of CO and unburnt contents, as well as to an increase in thermal loss.